Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
on systems where the run-time libraries distinguish between binary and
text files. If FILEHANDLE is an expression, the value is taken as the
name of the filehandle. DISCIPLINE can be either of
binary mode or
":crlf" for text mode. If the DISCIPLINE is
omitted, it defaults to
binmode() should be called after open() but before any I/O is done on
On many systems binmode() currently has no effect, but in future, it
will be extended to support user-defined input and output disciplines.
On some systems binmode() is necessary when you're not working with a
text file. For the sake of portability it is a good idea to always use
it when appropriate, and to never use it when it isn't appropriate.
In other words: Regardless of platform, use binmode() on binary
files, and do not use binmode() on text files.
open pragma can be used to establish default disciplines.
The operating system, device drivers, C libraries, and Perl run-time
system all work together to let the programmer treat a single
\n) as the line terminator, irrespective of the external
representation. On many operating systems, the native text file
representation matches the internal representation, but on some
platforms the external representation of
\n is made up of more than
Mac OS and all variants of Unix use a single character to end each line
in the external representation of text (even though that single
character is not necessarily the same across these platforms).
Consequently binmode() has no effect on these operating systems. In
other systems like VMS, MS-DOS and the various flavors of MS-Windows
your program sees a
\n as a simple
\cJ, but what's stored in text
files are the two characters
\cM\cJ. That means that, if you don't
use binmode() on these systems,
\cM\cJ sequences on disk will be
\n on input, and any
\n in your program will be
converted back to
\cM\cJ on output. This is what you want for text
files, but it can be disastrous for binary files.
Another consequence of using binmode() (on some systems) is that
special end-of-file markers will be seen as part of the data stream.
For systems from the Microsoft family this means that if your binary
\cZ, the I/O subsystem will ragard it as the end of
the file, unless you use binmode().
binmode() is not only important for readline() and print() operations,
but also when using read(), seek(), sysread(), syswrite() and tell()
(see perlport for more details). See the
in perlvar for how to manually set your input and output